Process for tanning leather



' Patented July 20, 1937 UNITED-- STATES rnocsss FOR TANNING LEATHER John Arthur Wilson,

' by mesne assignments,

Milwaukee, Wis., assignor,

to Monsanto Chemical Company, a corporation of Delaware No Drawing. Application May 10, 1935, Serial No.'20,'l69

8 Claims.

This invention relates to tanning processes and materials, and has for its object certain improvements in the tanning process. It has particular reference to an improved process for the tanning of sole leather, and, in addition, discloses certain new compositions valuable for tanning leather. In the tanning of heavy leather for sole or belting purposes the hides are subjected to tan liquors of gradually increasing strength, and 10 many weeks may be required to effect complete penetration of tannin and its fixation by the hide protein. Because the process is time consuming, the amount of material in process is always very large. It is highly desirable, therefore, to increase the rate of tanning where it can be done without any deleterious eifect to the produc I have now found that alkali metal polyphosphates, also referred to herein as molecularly 0 dehydrated alkali metal polyphosphates, may be used'in the tanning bath with great advantage as to the time of tanning and character of product obtained.

An important result of my improved process is a greatly accelerated rate of tanning by which an improved product may be obtained. Further objects and advantages will become apparent from the following disclosures.

I have prepared suitable tanning liquors in the following manner:-A series of liquors was made up containing, for example, equal amounts,

on a tannin basis, of wattle bark extract, cutch extract and spruce extract. The first liquor in the series contained 0.5% tannin, and each succeeding liquor higher by 0.5% tannin up to a value of 6.0% tannin in the head liquor. In the control series no phosphate was employed. In another series each liquor received 5 grams per liter of a sodium polyphosphate, specifically 4o soluble sodium metaphosphate. The pH values of the phosphate liquors were made the same as those of the controls by adding sulphuric acid.

The steer hides were then introduced into the weakest tanning liquor and twice each week 45 the hides were The hides were finally left in the strongest liquor until all had been completely struck through by the tannin, as shown by making daily cuttings of the hide.

The weakest liquors had an initial pH of 4.7.

The introduction of the hide always raises the pH value; with no phosphate it rose to 7.75, and with the metaphosphate it rose to 7.09. The strongest liquors had a pH of 3.68, the intermediate liquors 55 being graded about equally from 4.70 to 3.68.

moved to the next higher liquor.

The hide tanned in the control series required 44 days for the tanning to strike through the hide; with the metaphosphate present it required only 25 days.

The final degree of tannage of the control 5 leather was 48; for the series containing sodium metaphosphate, it was 44. However, the total load of organic matter carried into the hide to make leather was 104 for the metaphosphate series and '92 for the control series.

It is thus evident that the sodium polyphosphate speeds the tannin diffusion process and materially increases the amount of tanning material carried into the hides.

Those phosphates which I have found to be of 15 value in my process comprise the water-soluble alkali metal polyphosphates. The sodium salts, because of their cheapness and commercial availability, are preferred. The sodium polyphosphates, for example, are combinations of the 20 compound sodium metaphosphate alone or in molecular combination, or in chemical combination, with a pyrophosphate. These products may be made by mixing together, in varying proportions, the mono-alkali metal orthophosphate and 5 the di-alkali metal orthophosphate, heating the mixture to the fusion point, and quickly cooling the same. Under these conditions the orthophosphates are molecularly dehydrated and form polyphosphates. The manufacture of the sodium 30 polyphosphates in soluble form may be carried out as described in British patent 347,345, of 1932. According to this process a soluble sodium metaphosphate having a pH in 1% water solutions of approximately 6.3, or a series of soluble 35 sodium polyphosphates varying in pH- from this value to about 10.6, may be obtained and utilized in the present process. The product usually obtained commercially will contain the equivalent of, say, 80% sodium metaphosphate 40 and 20% tetra sodium pyrophosphate, together with small amounts-of sodium orthophosphate and acid sodium pyrophosphate.

- It is also evident that the proportions of polyphosphate employed in the tanning liquor may be varied in suitable manner, the above-mentioned compositions having been chosen as one example of a suitable concentration.

While I have described above certain specific forms of my invention, it will be appreciated that it is susceptible to various changes and modifications without departing from the spirit thereof, and I desire, therefore, that it be not limited except as indicated by the prior art, or as particularly pointed out in the claims.

What I claim is 1. A preparation for tanning hides comprising tannin and (a compound of alkali metal meta phosphate and alkali metal tetra pyrophosphate.

2. A tanning liquor comprising tannin-containing extracts and a compound of alkali metal meta phosphate and alkali metal tetra pyrophosphate.

3. A tanning liquor comprising tannin and a 10 compound of sodium meta phosphate and tetrasodium phyrophosphate.

4. A tanning preparation comprising tanninbearing extracts and sodium metaphosphate.

5. A tanning preparation comprising tannin- 15 bearing extracts and a sodium polyphosphate containing the equivalent of approximately 80% JOHN ARTHUR WILSON. 

